Wallboard is a generic term used in the building industry to define structure materials capable of being attached to wall or ceiling support structure and capable of receiving a decorative finish or design. Gypsum based wallboard products have heretofore been the standard of the industry, having gained acceptance due to their low cost and their generally adequate performance in the field. Because gypsum products are relatively dense, being in the range of 38 pcf to 45 pcf, they can be quite heavy, particularly in thicker widths of board. This makes it difficult to handle and install, and, because gypsum wallboard is heavy in relation to its strength, it has a tendency to break under its own weight when being handled. In fact, building materials dealers have reported up to 25% loss of the board due to damage incurred during handling.
Gypsum wallboard includes a family of interior wall and ceiling claddings comprising a core of hydrated gypsum rock and paper facers. The paper facers normally are made on cylinder machines from recycled paper in order to produce a porous paper capable of receiving the gypsum crystals that grow from the wet core slurry prior to drying. It is not practical to significantly decrease the density of the gypsum core in order to make a lighter weight product because with a core density below about 40 pcf, a good bond between the facer and core may not be obtained even when using optimum grade paper.
Obviously, it would be advantageous to have available a lightweight wallboard of sufficiently high strength to enable it to replace conventional gypsum wallboard. Such a product would provide a number of advantages over gypsum board. Because it would not be as heavy as gypsum board, but would nonetheless possess good strength, wallboard breakage would be greatly reduced. The lighter weight product would also decrease shipping costs and allow it to be carried by a single worker instead of two. One person could also install the board and could be counted on to work more efficiently for longer periods as a result of less job-caused fatigue. The likelihood of damage to predecorated wallboard would also be reduced because lighter weight wallboard would be easier to handle and therefore less likely to be brought into damaging contact with another surface.
The idea of binding a lightweight aggregate in order to make a board product is not new, and has in fact been suggested in the prior art. None of these suggestions, however, has resulted in a successful commercial product.
U.S. Pat. No. 3,819,388 to Cornwell discloses gypsum and portland cement based cementitious compositions which incorporate lightweight mineral aggregate such as vermiculite. The density of the product is controlled by the addition of a bubble forming additive which produces a cellular structure intended to provide for heat and sound insulation. The material is intended to be sprayed over steel structures or cast or pumped into wall structures. It was not designed to be formed into a board or panel to be used as a substitute for gypsum wallboard.
U.S. Pat. No. 4,042,406 to Gray discloses a building material for use in interior partitions of buildings. The material contains expanded perlite aggregate and portland cement, and is intended for use as a concrete block. The product has a higher density than desired, typically being about 41 pcf, the same as gypsum wallboard, and moreover is not designed for use as a wallboard.
U.S Pat. No. 4,263,048 to Hacker discloses a self-hardening cementitious composition, which may contain expanded aggregate as an extender, for use as an insulator against extremes of temperature. The composition is comprised of a combination of inorganic binders and related materials in order to make the product self-hardening. Expanded perlite up to 11.5 pcf in density can be incorporated in the mix. There is no suggestion in this patent of a simple, inexpensive lightweight high strength wallboard which can compete in terms of price and strength with gypsum board. Further, as will be brought out in more detail later, the present invention cannot employ expanded perlite of such high density.
U S. Patent No. 2,705,198 to Seybold discloses a wallboard containing fibrous material, mineral filler such as perlite, and a binder composition. Since the wallboard contains substantial amounts of fibrous material, a dilute slurry is employed in order to process the ingredients. The core mixture must therefore be dewatered before being compacted and dried to a board product. No densities or strength figures are given in the disclosure. Such a process cannot be used in connection with the present invention.
U.S. Pat. No. 4,126,512 to Hill discloses a lightweight roof insulating board comprising cellulosic fibers, expanded perlite particles and a binder comprised of asphalt and starch. The board product is designed to be used under a roof membrane, not as a wallboard, and is formed from an aqueous slurry comprising about 95% water, thereby requiring an expensive drying process to rid the wet composition of excess water.
U.S. Pat. No. 3,988,199 to Hillmer et al relates to the same general type of roof insulation board as discussed in the preceding paragraph, but includes gypsum as an additional essential ingredient. This is for the purpose of increasing the green strength of the composition during manufacture of the board so that the distance between the rollers which convey the wet board to the dryer can be maximized.
U.S. Pat. No. 4,695,494 to Fowler, Jr. et al discloses a fire door core material comprising a major amount of expanded perlite, adhesive material including starch, and glass fibers. The process of manufacture is a batch process which requires the dry ingredients, including the starch adhesive, to be mixed before adding water so that the ingredients and adhesive are ensured of being uniformly mixed prior to activating the adhesive. Further, the perlite particles are required to be in the density range of 4.5 pcf to 7.5 pcf in order to obtain the desired strength. As will be explained hereinafter, the present invention utilizes expanded perlite of a lower density range which surprisingly is required in order to reach the desired level of strength. The fire door core composition of Fowler, Jr. et al is not designed to be laminated to facer sheets in order to function as a wallboard.
U.S. Pat. No. 4,297,311 to Sherman et al discloses a mineral board product which comprises predominantly perlite with a resin binder. Fibrous additives may also be utilized if desired. The board is produced by a process which requires less water than the usual aqueous slurry type of forming process. In order to provide the desired level of strength the perlite is required to be crushed prior to the setting of the resin binder, which also reduces the bulk volume of the perlite. The perlite density may range from low to relatively high.
Although the above discussion illustrates the variety of products and processes known in the prior art, none of the prior art teachings is directed to a lightweight wallboard which is sufficiently strong and economical to be used in place of gypsum wallboard.